Shortening start of Arc/Curve within Complex geometry in ORD


I was hoping someone could help solve an issue I am having. Within Bentley rail track, the edit geometry tool was incredibly powerful and allowed manipulation of geometry to precise requirements (stationing for example). You could pretty much complete your design using this tool. It is a shame that is no longer included.

I am trying to shorten the START of a curve, so that the end of the curve is fixed. Whenever I try to use the manipulators, it changes the radius of the curve also which is not what i am looking for. Is there a way to shorten the length of the curve using precise methods without changing anything else? For example, I want to shorten a curve of 185.205m to 180m, whilst maintaining the radius of 1216.015 and maintaining its connection to the following spiral.

This is particularly useful for ensuring the design starts at the correct stationing and to cut the design to a position whereby it ties into existing at an appropriate position.